The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A propeller operates by generating a thrust to drive a body, such as an aircraft or boat, by applying a force to the fluid in which it operates in order to change the momentum of the fluid in the direction opposite to that in which it is desired to drive the body. Each blade of the propeller, screw etc. is inclined to the desired direction of thrust, so that, as it rotates through the fluid, the normal reaction force has a component parallel to the direction of drive, the orientation being set to ensure that this force tends to push the fluid in a direction opposite to the desired direction of travel. Under Newton's first law, action and reaction are equal and opposite, and hence the fluid applies an equal and opposite force to the screw, propeller, etc. which reaction force is transmitted to the body and in turn drives the body through the fluid.
Conventional screw design is based on the works of Archimedes (c. 250 BC) and was later modified by Francis Petit Smith, and a typical such marine screw comprises a number of blades, normally 2 to 4, which are fixed to a hub either with their longitudinal axis perpendicular to the axis of the hub or inclined thereto towards the back of the craft on which the propeller is attached so as to produce aft rake. The blades are attached to the hub in a symmetrical pattern and each blade typically has a curved outer profile, normally formed by ogival or aerofoil sections, tapers outwardly from root to tip and twist from root to tip so that the tip of the blade has a greater angle of attack to the incident fluid than the root.
This conventional configuration has, however, a number of disadvantages. The shape of the blades tends to generate lift, and the resulting pressure differences can lead to early cavitation. Also, the twist or skew on a typical blade, which results in a variation in pitch along the blade, makes the propeller more complicated and hence more expensive to produce. Whilst attempts have been made over the years to reduce the impact of these drawbacks and also to improve efficiency, reduce cavitation etc of propellers, any improvements that have been achieved have been relatively small, and involved minor variations in blade shape, twist, size and taper without changing the underlying design of the marine screw.
British patent application no. 0411155.5 discloses a propeller blade design comprising a first blade portion having an arcuate cross section and being in the form of a longitudinal segment of a hollow truncated cone defined between two planes which extend longitudinally of the cone, are inclined to each other and which each intersect each other along the longitudinal axis of the cone, and a second blade section which extends adjacent the first blade portion, which is of similar shape to the first blade section with the radius of curvature of the two blade portions being the same at each point along the blade, and the first and second portions being arranged side by side facing opposite directions so as to form a sigmoid cross section.